The medical error report from the Institute of Medicine (1999) has greatly increased people's awareness of the frequency, magnitude, complexity, and seriousness of medical errors. As the eighth leading cause of death in US, ahead of motor vehicle accidents, breast cancer, and AIDS, medical errors need immediate attention from academic, health care, and government organizations. While medical errors can be dealt with from several perspectives, such as organizational restructuring or automation, our concern is with cognitive factors of medical errors and their clinical implications. We propose a theoretical framework in which errors are viewed as inevitable but cognitively useful phenomena that cannot be totally eliminated. In our view human errors are products of cognitive activities in people's adaptation to their complex physical, social, and cultural environments. Our cognitive approach stresses actions in conceptual understanding and thought processes during clinical problem solving. The actions reflect the level of expertise and the demands of tasks in clinical performance. In order to manage errors during clinical decision-making, it is critical to understand how decisions are made and what underlying cognitive mechanisms are used to process information during interactions with patients, colleagues, and technology in the health care environment. The broad objective of the proposed research is to develop a cognitive framework of medical errors in critical care environments (medicine, surgery and psychiatry), where decisions are often made under high stress, time pressure, and with incomplete information. Our specific aims include (1) applying and refining our initial cognitive taxonomy of errors where each category of medical error is associated with a specific cognitive mechanism, and (2) providing a theoretical explanation of why these errors occur and predicting the circumstances in which such errors would occur. Unlike the popular goal of achieving flawless performance (through development of error-free systems), the results from this study will have implications for developing adaptive systems that anticipate errors, respond to them, or substitute less serious errors that allow subsequent intervention before they result in an adverse event.